This invention relates to a method of fabricating a semiconductor device having high density, high speed, and low power consumption.
In a semiconductor integrated circuit, higher density, higher speed, and lower power consumption have been sought, and various attempts have been made in the development of devices of SOI (silicon-on-insulator) structure, to obtain higher density by decrease of the separation region between devices, or higher speed and lower power consumption by decrease of parasitic capacity generated between source-drain and substrate.
For example, Japanese Laid-open patent No. 54-88871 discloses a fabricating process of SOI structure semiconductor device of ditch separation development type. In this process, in the first place, a selectively opened thick Si.sub.3 N.sub.4 film is formed on a silicon substrate. Then, using this Si.sub.3 N.sub.4 film as a mask, an opening is formed in the silicon substrate by a strongly anisotropic dry etching method, for example, reactive ion etching (R.I.E.). To this steep opening plane of this opening part, a thin Si.sub.3 N.sub.4 film is deposited by reduced pressure CVD process. And the surface thin Si.sub.3 N.sub.4 film is removed by sputter etching method. Since the sputter etching method is excellent in the linearity of etching, the thin Si.sub.3 N.sub.4 film at the side of the opening part is not etched, and only the upper surface of thick Si.sub.3 N.sub.4 film and the thin Si.sub.3 N.sub.4 film on the bottom of the silicon substrate opening part are etched. Afterwards, the silicon substrate is etched and oxidized, and an oxide region is formed in the silicon substrate, so that the entire lower surfaces of monocrystalline silicon insular regions located between adjacent opening parts are communicated by oxidation from both sides. Later, when the Si.sub.3 N.sub.4 film is removed from the surface of monocrystalline silicon insular region, the lower face and side face of the monocrystalline silicon insular region are surrounded entirely by the oxide region.
The above etching of silicon substrate must be done isotropically from the viewpoint of stabilization of the shape of silicon insular region by oxidation process and decrease of defects in the silicon insular region due to shortening of oxidation time. In a similar fabricating method of SOI structure semiconductor device of ditch separation development type (Japanese Laid-open patent No. 58-250429), this isotropic etching is effected, for example, by wet process.
In the wet process etching, however, as the density becomes higher, the separation region between adjacent silicon islands becomes narrower, and the liquid is hard to penetrate. As a result, the etching shape may not be uniform, and some parts of the silicon island bottom are oxidized and others are not.